The Thames
in which the stream swings from one side to the other, and even where it touches higher soil, touches nothing better than the continuation of this clay. In spite, therefore, of the shallowness and narrowness of the upper river there always existed this impediment which an insecure soil would present to the formation of any considerable settlements. The loneliness of the stretch below Kelmscott is due to an original difficulty of this kind, and the one considerable settlement upon the upper river at Lechlade stands upon the only place where firm ground approaches the bank of the river.
This formation endures well below Oxford until one reaches the gap at Sandford, where the stream passes between two beds of gravel which very nearly approach either bank.
Above this point the Thames is everywhere, upon one side or the other, guarded by flat river meadows, which must in early times have been morass; and nowhere were these more difficult of passage than in the last network of streams between Witham Hill and Sandford, to the west of the gravel bank upon which Oxford is built.
Below Sandford, and on all the way to London Bridge, the character of the river in this respect changes. You have everywhere gravel or flinty chalk, with but a narrow bed of alluvial soil, upon either bank to represent the original overflow of the river.
At the crossing places (as we shall see later), notably at
